A basic component of a magnetic tunnel junction (MTJ) is a sandwich of two thin ferromagnetic layers separated by a very thin insulating layer through which electrons can tunnel. The tunneling resistance is often lower when the magnetic moments of the ferromagnetic layers are parallel and higher when the magnetic moments of the two ferromagnetic layers are anti-parallel.
The change in conductance for these two magnetic states can be described as a magneto-resistance. The tunneling magneto-resistance (TMR) of the MTJ can be defined as (RAP−RP)/RP where RP and RAP are the resistance of the MTJ for parallel and anti-parallel alignment of the ferromagnetic layers, respectively. MTJ devices have been proposed as memory cells for nonvolatile solid state memory and as external magnetic field sensors, such as TMR read sensors for heads for magnetic recording systems. For a memory cell application, one of the ferromagnetic layers in the MTJ is the reference layer and has its magnetic moment fixed or pinned via a synthetic ferromagnetic (SAF) layer and an anti-ferromagnetic (AFM) layer, so that its magnetic moment is unaffected by the presence of the magnetic fields applied to the device during its operation. The other ferromagnetic layer in the sandwich is the free layer, whose moment responds to an external magnetic field applied during operation of the device. In the quiescent state, in the absence of any applied magnetic field within the memory cell, the free layer magnetic moment is designed to be either parallel (P) or anti-parallel (AP) to the magnetic moment of the reference ferromagnetic layer. For a TMR field sensor for read head applications, the reference ferromagnetic layer has its magnetic moment fixed or pinned via a synthetic ferromagnetic (SAF) layer and an anti-ferromagnetic (AFM) layer so as to be generally perpendicular to the magnetic moment of the free or sensing ferromagnetic layer in the absence of an external magnetic field.
For applications of magnetic tunnel junctions for either magnetic recording heads or for non-volatile magnetic memory storage cells, high TMR values are desired for improving the performance of these devices.